block: Add bio req flag to disable encryption in block

When lower layers such as dm-crypt observe the REQ_NOENCRYPT flag, it
helps the I/O stack avoid redundant encryption, improving performance
and power utilization.

Note that lower layers must be consistent in their observation of this
flag in order to avoid the possibility of data corruption.

Change-Id: I4805c3b06b3e765d7362d0b360523813ed77b8aa
Signed-off-by: Michael Halcrow <mhalcrow@google.com>
Git-repo: https://android.googlesource.com/kernel/common/


Git-commit: a0948fba1f34e8fc2b689d6928283b3f64158923
[stummala@codeaurora.org: resolve merge conflicts]
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>